1. Field of the Invention
The present invention relates to a semiconductor storage device having a sense amplifier circuit, and adopting an overdrive method to improve operation speed.
Priority is claimed on Japanese Patent Application No. 2007-182539, filed on Jul. 11, 2007, the contents of which are incorporated herein by reference.
2. Description of Related Art
In conventional semiconductor storage devices such as a dynamic random access memory (DRAM), a sense amplifier performs amplification and rewrite operations of a minute potential difference output to bit lines from a memory cell which stores data. In order to boost the operating speed of sense amplifiers, a method called an overdrive method has been adopted. In the overdrive method, the common source of a PMOS (P-channel metal oxide semiconductor) transistor provided in a sense amplifier is increased to an overdrive voltage, at a potential higher than the write potential (VARY potential or array potential) of a memory cell, to accelerate the sense operation (see Japanese Unexamined Patent Application, First Publication No. 2006-196123 (hereafter referred to as “Patent Reference 1”)) . There are a method which employs the voltage of a step-down power supply generated by a power supply generation circuit within the semiconductor storage device as the overdrive voltage, and a method which directly utilizes an external power supply voltage as the overdrive voltage.
In the normal overdrive method, the overdrive time and VARY power supply capacitance are adjusted such that the final write potential for memory cells is assured. If the overdrive voltage is VOD, the final write potential of the memory cells is VARY, the VARY power supply capacitance is Ca, the bit line parasitic capacitance is Cb, the memory cell capacitance is Cs, and the number of sense amplifiers activated simultaneously is n, then the generally employed VARY/2 precharge-type DRAM devices set the VARY power supply capacitance Ca and the overdrive voltage VOD such that the following equation conforming to the charge conservation law obtains.Ca×VOD+{(Cs+Cb)×n}×VARY/2={Ca+(Cs+Cb)×n}×VARY
In this equation, only the VARY power supply capacitance Ca and the overdrive voltage VOD can be regarded as variables, and the resistor R between power supply lines and overdrive time ta, which are parameters disclosed in embodiments of the present invention described below, are not present. According to this equation, a relation obtains in which in order to reduce the VARY power supply capacitance Ca, the overdrive voltage VOD must be made high, and in order to lower the overdrive voltage VOD, the VARY power supply capacitance Ca must be increased.
In successive generations of DRAM itself, power supply voltages have been lowered to reduce power consumption, and it has become impossible to set a high overdrive voltage VOD. On the other hand, miniaturization has been accompanied by smaller chip areas, and it has become difficult to secure the area necessary for the required VARY power supply capacitance Ca.
In overdrive methods adopted by conventional semiconductor storage devices, there is the problem that a high overdrive power supply potential or a large overdrive capacitance is necessary. The reason for this problem is that overdrive is performed primarily by electric charge accumulated in the overdrive capacitor. If the capacitance value of a capacitance element such as a VARY power supply capacitor is C0, and the potential difference across the ends of the capacitance element is V, then the amount of charge q accumulated in the capacitance element is given by the product of the capacitance value and the potential difference across the ends.q=C0×V
Hence in order to obtain a constant charge q, the only options are to increase either the capacitance value C0 or the potential difference V. However, due to advances in miniaturization and reduction of power consumption, both the capacitance value C0 and the potential difference V are small. Consequently, the amount of charge q is also small, and appropriate overdrive operation is no longer possible.